Add pattern match in match.pd for .AVG_CEIL

Add pattern match in match.pd for .AVG_CEIL

1) Optimize (a >> 1) + (b >> 1) + ((a | b) & 1) to .AVG_CEIL (a, b)
2) Optimize (a | b) - ((a ^ b) >> 1) to .AVG_CEIL (a, b)

gcc/ChangeLog:

	PR middle-end/118994
	* match.pd ((a >> 1) + (b >> 1) + ((a | b) & 1) to
	.AVG_CEIL (a, b)): New pattern.
	((a | b) - ((a ^ b) >> 1) to .AVG_CEIL (a, b)): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr118994-1.c: New test.
	* gcc.target/i386/pr118994-2.c: New test.